Final answer:
It is false that the switch from IgM to IgD is mediated by DNA rearrangements. The DNA rearrangements involve class switching in mature B cells, resulting in the production of different antibody classes while retaining the original antigen specificity.
Step-by-step explanation:
The switch in constant region use from IgM to IgD is false. The process you're referring to, known as class switching or isotype switching, is actually mediated by DNA rearrangements that occur within a mature B cell after it has been activated. This involves the genetic rearrangement of gene segments encoding the constant region of the antibody molecule. Class switching allows a B cell to produce different antibody classes, such as IgG, IgA, or Ige, that still retain the original variable region's epitope specificity. This variability in antibody responses is crucial for the immune system to effectively and efficiently react to a wide range of antigens.